The present invention relates to devices and methods for microwave-assisted chemistry. As generally recognized in the chemical arts, many chemical reactions can be initiated or accelerated by increasing the temperature—i.e. heating—the reactants. Accordingly, carrying out chemical reactions at elevated (i.e., above ambient) temperatures is a normal part of many chemical processes.
For many types of chemical compositions, microwave energy provides an advantageous method of heating the composition. As is well recognized in the art, microwaves are generally categorized as having frequencies within the electromagnetic spectrum of between about 1 gigahertz and 1 terahertz, and corresponding wavelengths of between about 1 millimeter and 1 meter. Microwaves tend to react well with polar molecules and cause them to rotate. This in turn tends to heat the material under the influence of the microwaves In many circumstances, microwave heating is quite advantageous because microwave radiation tends to interact immediately with substances that are microwave-responsive, thus raising the temperature very quickly. Other heating methods, including conduction or convection heating, are advantageous in certain circumstances, but generally require longer lead times to heat any given material.
In a similar manner, the cessation of application of microwaves causes an immediate corresponding cessation of the molecular movement that they cause. Thus, using microwave radiation to heat chemicals and compositions can offer significant advantages for initiating, controlling, and accelerating certain chemical and physical processes.
In recent years, much interest in the fields of chemical synthesis and analysis has focused upon the use, synthesis or analysis of relatively small samples. For example, in those techniques that are generally referred to as “combinatorial” chemistry, large numbers of small samples are handled (e.g., synthesized, reacted, analyzed, etc.) concurrently for the purpose of gathering large amounts of information about related compounds and compositions. Those compounds or compositions meeting certain threshold criteria can then be studied in more detail using more conventional techniques.
Handling small samples, however, tends to present difficulties in conventional microwave-assisted instruments. In particular, small masses of material are generally harder to successfully affect with microwaves than are larger masses. As known to those of ordinary skill in this art, the interaction of microwaves with responsive materials is referred to as “coupling.” Thus, stated differently, coupling is more difficult with smaller samples than with larger samples.
Furthermore, because of the nature of microwaves, specifically including their particular wavelengths and frequencies, their interaction with particular samples depends upon the cavity into which they are transmitted, as well as the size and type of the sample being heated.
Accordingly, in order to moderate or eliminate coupling problems, conventional microwave techniques tend to incorporate a given cavity size, a given frequency, and similarly sized samples. Such techniques are useful in many circumstances and have achieved wide acceptance and use. Nevertheless, in other circumstances when one of these parameters—sample size, material, microwave frequency—is desirably or necessarily changed, the cavity typically has to be re-tuned in order to provide the appropriate coupling with the differing loads. Stated somewhat differently, and by way of illustration rather than limitation, in a conventional device a one gram load would require tuning different from a ten gram load, and both of which would require different tuning from a hundred gram load, and all of which would differ if the microwave frequency or type of material is changed.
As another issue, differently-sized samples are generally most conveniently handled in reaction vessels that are proportionally sized based on the size of the sample. Many instruments for microwave-assisted chemistry, however, are—for logical reasons in most cases—made to handle vessels of a single size; e.g. instruments such as described in U.S. Pat. No. 5,320,804 or open vessels as described in U.S. Pat. No. 5,796,080. Thus although such instruments are valuable for certain purposes, the are generally less convenient, and in some cases quite ineffective for samples, vessels, and reaction other than a certain size (volume) or type.
As yet another issue, many reactions proceed more favorably under increased (i.e. above atmospheric) pressure. Controlling and using increased pressures for small samples in microwave-assisted chemistry can, for the reasons stated above and others, be somewhat difficult.
Accordingly, the need exists for new and improved instruments for microwave assisted chemistry that can handle small samples, can conveniently handle a variety of sample sizes and vessel sizes and that can incorporate and handle higher pressure reactions when desired or necessary.